Often used in process measurements and automation, technology for measuring physical parameters, such as e.g. mass flow, density and/or viscosity, of media flowing in pipelines are measuring systems, most often in the form of in-line measuring devices of compact construction, which, by means of a measuring transducer of vibration-type, through which medium flows, and a measuring, and driver, circuit connected thereto, effect, in the medium, reaction forces, such as e.g. Coriolis forces corresponding with mass flow of the medium, inertial forces corresponding with density of the medium and/or frictional forces corresponding with viscosity of the medium, etc., and produce, derived from these, a measurement signal representing the particular mass flow, viscosity and/or density of the medium. Such measuring transducers, especially measuring transducers embodied as Coriolis, mass flow meters or Coriolis, mass flow/densimeters, are described in detail e.g. in U.S. Pat. No. 4,793,191, U.S. Pat. No. 4,823,614, U.S. Pat. No. 4,831,885, U.S. Pat. No. 5,602,345, US-A 2007/0151368, US-A 2010/0050783, WO-A 96/08697, WO-A 2009/120222 or WO-A 2009/120223.
Measuring transducers of the type being discussed here include a transducer housing, of which an inlet-side, first housing end is formed at least partially by means of a first flow divider having exactly two, mutually spaced, circularly cylindrical, or tapered or conical, flow openings and an outlet-side, second housing end is formed at least partially by means of a second flow divider having exactly two, mutually spaced, flow openings. As shown in U.S. Pat. No. 5,602,345 or WO-A 96/08697, the transducer housing can be formed, for example, by means of a circularly cylindrical tube, in given cases, also a rather thick-walled, circularly cylindrical tube.
For guiding the medium, which flows, at least at times, each of the aforementioned measuring transducers includes, furthermore, in each case, two or more—in US-A 2010/0050783, or WO-A 96/08697, namely, exactly four straight, measuring tubes, which are connected such that the medium can flow in parallel. The measuring tubes are made of metal, for example titanium, zirconium or tantalum and positioned within the transducer housing and held oscillatably therein by means of the aforementioned flow dividers.
In the case of measuring transducers shown in US-A 2010/0050783 or WO-A 96/08697 with exactly four, equally-constructed, straight measuring tubes extending parallel relative to one another, a first of the measuring tubes opens with an inlet-side, first, measuring tube end into a first flow opening of the inlet-side, first flow divider and with an outlet-side, second measuring tube end into a first flow opening of the outlet-side, second flow divider, a second of the measuring tubes opens with an inlet-side, first measuring tube end into a second flow opening of the first flow divider and with an outlet-side, second measuring tube end into a second flow opening of the second flow divider, a third of the measuring tubes opens with an inlet-side, first, measuring tube end into a third flow opening of the inlet-side, first flow divider and with an outlet-side, second measuring tube end into a third flow opening of the outlet-side, second flow divider, and a fourth of the measuring tubes opens with an inlet-side, first measuring tube end into a fourth flow opening of the first flow divider and with an outlet-side, second measuring tube end into a fourth flow opening of the second flow divider. Each of the flow dividers includes additionally, in each case, a flange with a sealing surface for fluid tight connecting of the measuring transducer to tubular segments of the pipeline serving, respectively, for supplying and for removing medium to and from the measuring transducer.
For producing the above discussed reaction forces, the measuring tubes of vibration type are caused to vibrate during operation, about their respective static rest positions, driven by an exciter mechanism serving for producing, or maintaining, as the case may be, mechanical oscillations, especially bending oscillations, of the measuring tubes in the so-called wanted mode. The oscillations in the wanted mode are, most often, especially in the case of application of the measuring transducer as a Coriolis, mass flow meter and/or densimeter, developed, at least partially, as lateral bending oscillations and, in the case of medium flowing through the measuring tubes, as a result of therein induced Coriolis forces, as additional, equal frequency oscillations superimposed in the so-called Coriolis mode. Accordingly, the—here most often electrodynamic—exciter mechanism is embodied in such a manner, that, therewith, the two measuring tubes—in the case of four measuring tubes, in each case, two of the measuring tubes of one of the two measuring tube pairs—are excitable in the wanted mode, at least partially, especially also predominantly, to opposite equal bending oscillations in a common plane of oscillation—most often, the tubes are excitable differentially, thus through introduction of exciter forces acting simultaneously along a shared line of action, however, in opposed directions.
For registering vibrations of the measuring tubes, especially also the oscillations actively excited by means of the exciter mechanism in the wanted mode and for producing oscillation measurement signals representing vibrations, the measuring transducers have, additionally, in each case, a, most often, likewise electrodynamic, vibration sensor arrangement reacting to relative movements of the measuring tubes. Typically, the vibration sensor arrangement is formed by means of an inlet-side, oscillation sensor registering oscillations of the measuring tubes, as, among other things, also shown in US-A 2010/0050783, or WO-A 96/08697, differentially—thus only relative movements of the measuring tubes—as well as by means of an outlet-side, oscillation sensor also registering oscillations of the measuring tubes differentially. Each of the oscillation sensors, which are usually constructed equally with one another, is, in such case, formed by means of a permanent magnet held on the first measuring tube and a cylindrical coil held on the second measuring tube and permeated by the magnetic field of the permanent magnet.
In operation, the above described inner part of the measuring transducer, here, thus, an inner part formed by means of the two, or four, measuring tubes and the exciter mechanism and vibration sensor arrangement held thereto, is excited by means of the electromechanical exciter mechanism, at least at times, to execute mechanical oscillations in the wanted mode at least one dominating, wanted, oscillation frequency. Selected as oscillation frequency for the oscillations in the wanted mode is, in such case, usually a natural, instantaneous, resonance frequency of the inner part, which, in turn, depends essentially both on size, shape and material of the measuring tubes as well as also on an instantaneous density of the medium; in given cases, this wanted oscillation frequency can also be influenced significantly by an instantaneous viscosity of the medium. As a result of fluctuating density of the medium being measured and/or as a result of media change occurring during operation, the wanted oscillation frequency during operation of the measuring transducer varies naturally, at least within a calibrated and, thus, predetermined, wanted frequency band, which correspondingly has a predetermined lower, and a predetermined upper, limit frequency.
For defining a wanted oscillation length of the measuring tubes and, associated therewith, for adjusting the band of the wanted frequency, measuring transducers of the above described type include, additionally, most often, at least one inlet-side, coupling element, which is affixed to both measuring tubes and spaced from the two flow dividers, for forming inlet-side, oscillation nodes for opposite equal vibrations, especially bending oscillations, of the measuring tubes, as well as at least one outlet-side, coupling element, which is affixed to both measuring tubes and spaced both from the two flow dividers, as well as also from the inlet-side, coupling element, for forming outlet-side, oscillation nodes for opposite equal vibrations, especially bending oscillations, of the measuring tubes. In such case, a minimum separation between inlet-side and outlet side coupling elements, which thus also belong to the inner part, corresponds to the wanted oscillation length of the measuring tubes. By means of the coupling elements, additionally also an oscillation quality factor of the inner part, as well as also the sensitivity of the measuring transducer, in total, can be influenced, in a manner such that, for a minimum required sensitivity of the measuring transducer, at least one minimum, wanted oscillation length is provided.
Development in the field of measuring transducers of vibration-type has, in the meantime, reached a level, wherein modern measuring transducers of the described type can, for a broad application spectrum of flow measurement technology, satisfy highest requirements as regards precision and reproducibility of the measurement results. Thus, such measuring transducers are, in practice, applied for mass flow rates from some few g/h (gram per hour) up to some t/min (tons per minute), at pressures of up to 100 bar for liquids or even over 300 bar for gases. The accuracy of measurement achieved, in such case, lies usually at about 99.9% of the actual value, or above, or at a measuring error of about 0.1%, wherein a lower limit of the guaranteed measurement range can lie quite easily at about 1% of the measurement range end value. Due to the high bandwidth of their opportunities for use, industrial grade measuring transducers of vibration-type are available with nominal diameters (corresponding to the caliber of the pipeline to be connected to the measuring transducer, or corresponding to the caliber of the measuring transducer measured at the connecting flange), which lie in a nominal diameter range between 1 mm and 250 mm and at maximum nominal mass flow rate of 2200 t/h, in each case, for pressure losses of less than 1 bar. A caliber of the measuring tubes lies, in such case, for instance, in a range between 80 mm and 100 mm.
In spite of the fact that, in the meantime, measuring transducers for use in pipelines with very high mass flow rates and, associated therewith, very large calibers of far beyond 100 mm have become available, there is still considerable interest in obtaining measuring transducers of high precision and low pressure loss also for yet larger pipeline calibers, about 300 mm or more, or mass flow rates of 2500 t/h or more, for instance for applications in the petrochemical industry or in the field of transport and transfer of petroleum, natural gas, fuels, etc. This would, in the case of correspondingly scaled enlarging of the already established measuring transducer designs with two measuring tubes, as known from the state of the art, for instance, the above mentioned U.S. Pat. No. 4,793,191, U.S. Pat. No. 5,602,345, US-A 2007/0151368, WO-A 2009/120222, or WO-A 2009/120223, lead to the fact that the geometric dimensions would be exorbitantly large, especially the installed length corresponding to a distance between the sealing surfaces of both flanges, especially dimensions for the desired oscillation characteristics, the required mechanical load bearing ability (especially also for preventing possible deformations of the measuring transducer significant for the oscillatory behavior of the measuring tubes), as well as the maximum allowed pressure loss. Along with that, also the empty mass of the measuring transducer increases unavoidably, with conventional measuring transducers of large nominal diameter already having an empty mass of about 400 kg. Further investigations, which have been carried out for measuring transducers with two straight measuring tubes, constructed as regards their to-scale enlargement to still greater nominal diameters, have, for example, shown that, for nominal diameters of more than 300 mm, the empty mass of a to-scale enlarged, conventional measuring transducer would lie far above 600 kg, accompanied by an installed length of more than 3000 mm.
In the case of application of four straight measuring tubes, as a result of the measurement range end value, or expanded measurement range, increased as a result of the type of construction, it is true that, at first, indeed, a certain amount of reduction of the installed length of the measuring transducer can be achieved at equal nominal diameter, or equal pressure loss brought about by the measuring transducer. Insofar, thus a more powerful measuring system can be provided than is possible with established measuring transducers with two measuring tubes at comparable technical effort. However in the case of the measuring transducer concepts known from the state of the art with four straight measuring tubes, at least in the case of greater nominal diameters of over 100 mm, there is regretfully a comparatively higher (cross-) sensitivity of the zero-point for mass flow measurement to mechanical alternating loads, consequently pipeline vibrations or pressure fluctuations, introduced externally via the pipeline into the measuring transducer. This increased “mechanical” zero-point sensitivity is especially to be attributed to the facts that there is, in comparison to measuring transducers with two measuring tubes of equal nominal diameter, or comparable flow resistance, on the one hand, a larger contact area, via which oscillations of the measuring tubes are externally effectively influenceable—here, namely, practically the entire cross section of each of the flow dividers flowed through by medium during operation —, and, on the other hand, as a result of the flow dividers being more flexible in comparison to conventional flow dividers there is a more effective coupling created for outer mechanical disturbances. An opportunity to remove this increased zero-point sensitivity is then correspondingly to stiffen the transducer housing, consequently to increase its wall thicknesses and/or to provide additional stiffening elements. To achieve this, it would, however, be necessary also to bring back increased empty mass of the measuring transducer in considerable measure, consequently a mass to nominal diameter ratio of the measuring transducer, defined by a ratio of the empty mass of the measuring transducer to the nominal diameter of the measuring transducer, worsens, and, indeed, in such a measure that measuring transducers with four straight measuring tubes are no longer mentionably better than conventional measuring transducers, and can, in fact, even be worse.